The invention relates to tunable laser systems, particularly of the type having adjustable external cavities for varying the wavelength of laser output beams. One end of such cavities is formed by diffractive optics that are inclined to the laser beams under the control of adjustment mechanisms to provide controlled wavelength feedback.
External cavity lasers are well known in the art. The external cavity returns a portion of the radiation generated by the laser back into a primary laser cavity as a form of optical feedback that alters the laser radiation being amplified in the primary laser cavity. Modifications can be made to the external cavity to control properties of the light amplified in the primary laser cavity. This procedure has been used for several years to stabilize and tune the frequency output of dye lasers and is described in Spectrally Narrow Pulsed Dye Laser Without Beam Expander, Applied Optics, Vol. 17, No. 14, Jul. 15, 1978. External cavities are also commonly used with diode lasers and can be used to create narrowband, wavelength tunable diode laser systems for such applications as telecommunications, spectroscopy, and metrology.
A primary laser has a defined output beam direction and lasing wavelength. The lasing wavelength, which is the wavelength that exhibits maximum gain, is set by properties of the laser gain medium and the laser cavity. While it is possible for the primary laser to operate at any of a range of wavelengths, a single wavelength can be favored by prevailing conditions for which gain is maximized.
The external cavity allows for the alteration of the lasing wavelength of the primary laser by providing feedback to the laser in the form of a selected wavelength other than the one initially favored by the primary laser. In this manner, the primary laser is artificially caused to favor another wavelength, thus creating a means to tune the wavelength of the laser system.
Two types of external laser cavities are commonly usedxe2x80x94the Littman cavity and the Littrow cavity. Specific configurations of Littman cavities have been the subject of U.S. Pat. Nos. 5,319,668; 5,802,085; and 5,867,512. An article in the March 1998 Review of Scientific Instruments, Volume 69, Number 3, pages 1236-1239, by Arnold et al. entitled xe2x80x9cA Simple Extended-cavity Diode Laserxe2x80x9d describes an exemplary Littrow cavity configuration. These patents and the above referenced articles are incorporated by reference.
Although both the Littman cavity and Littrow cavity configurations provide effective wavelength tuning, the Littrow cavity configuration is preferred for many commercial applications because it has a lower component cost. According to the Littrow configuration, a collimated output beam from the primary laser is directed toward a reflective diffraction grating that is oriented so that a first diffracted order of the output beam is returned to the primary laser on a path of retroreflection. The wavelength of the first diffracted order provides feedback to the laser to influence the lasing wavelength. The zero diffracted order of the output beam reflects from the grating for exiting the external cavity.
For returning a particular wavelength xe2x80x9cxcexxe2x80x9d, a Littrow orientation angle xe2x80x9cxcfx86pitchxe2x80x9d of the grating normal with respect to the beam direction of the primary laser is given by       φ    pitch    =            sin              -        1              ⁢          xe2x80x83        ⁢          (              λ                  2          ⁢          d                    )      
where xe2x80x9cdxe2x80x9d is the grating period. The external cavity thus provides feedback at wavelength xe2x80x9cxcexxe2x80x9d; and if this feedback is such that xe2x80x9cxcexxe2x80x9d becomes the wavelength for which the primary laser has maximum gain, then the laser system will lase at the feedback wavelength xe2x80x9cxcexxe2x80x9d.
A difficulty with the Littrow configuration, as shown in FIG. 1, is that the orientation of an output beam 18 from the external cavity 16 varies as the pitch angle xcfx86pitch of the diffraction grating 14 is changed. Thus, as the output beam 18 is tuned to different wavelengths, the orientation of the output beam changes, which produces significant alignment problems for optical devices intended for operation over multiple wavelengths.
The invention contemplates a wavelength tuning system for adjusting the external cavity of lasers and is intended to be particularly effective for varying output beam wavelength while maintaining the output beam from the external cavity at a fixed angular orientation. Improvements are made to the adjustment and alignment of components as well as to the components themselves, resulting in a robust structure that is easy to align and to maintain in alignment. The invention is especially useful for providing a tunable external cavity laser that operates optimally for various applications, such as multi-wavelength interferometry.
An exemplary wavelength tuning system for adjusting the external cavity of a laser in accordance with the invention includes a laser mount for supporting the laser that emits a wavelength-tunable output beam from the external cavity. A mounting arm supports both a diffractive optic and a reflective optic in a fixed orientation with respect to each other. A flexural member forms at least part of a connection between the mounting arm and the laser mount in a relative orientation that optically couples the diffractive optic to the laser within the external cavity. An actuator pivots the mounting arm with respect to the laser mount about a pivot axis for varying a wavelength of a diffracted portion of the output beam returned to the laser without significantly varying an angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic. The flexural member supports a limited rotation of the mounting arm with respect to the laser mount about the pivot axis while providing resistance to a similar limited rotation of the mounting arm with respect to the laser mount about an orthogonal rotational axis.
Preferably, the tuning system also includes an adjuster to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the orthogonal rotational axis. One such adjuster provides an adjustable mounting for the flexural member that adjusts the mounting of the flexural member between the mounting arm and the laser mount about the orthogonal rotational axis.
The orthogonal rotational axis is preferably a first of two orthogonal rotational axes that are orthogonal to the pivot axis, and the adjuster is preferably a first of two adjusters that adjust the relative angular orientation of the mounting arm with respect to the laser mount. A second of the adjusters can be used to adjust the relative angular orientation of the mounting arm with respect to the laser mount about the second of the orthogonal rotational axes. For example, the second adjuster can provide an adjustment of the flexural member between the mounting arm and the laser mount about the second orthogonal rotational axis. Similar adjustments can be made between two portions of the mounting arm, a first portion being connected to the flexural member and a second portion supporting both the diffractive optic and the reflective optic.
Alternatively, the second adjuster could engage the mounting arm remote from the flexural member for imparting a limited angular adjustment of the mounting arm about the second orthogonal rotational axis with respect to the laser mount through the flexural member. The mounting arm of this alternative preferably includes first and second endsxe2x80x94the first end being connected to the flexural member and a second end being connected to the second adjuster. The second adjuster permits motion of the mounting arm about the pivot axis while restricting further motion about the second orthogonal rotational axis from an established adjustment position. The flexural member preferably exerts a first preload torque on the mounting arm about the pivot axis for imparting a preload force on the actuator and preferably exerts a second preload torque on the mounting arm about the second orthogonal rotational axis for imparting a preload force on the second adjuster.
The first and second adjusters provide for adjusting the diffractive optic to return the diffracted portion of the output beam to the laser along a path of retroreflection. The diffractive optic preferably has a planar surface with rulings, and the first and second adjusters provide for relatively adjusting the rulings substantially parallel to the pivot axis. In this regard, the first adjuster preferably provides for relatively adjusting the rulings about the first orthogonal rotational axis in an orientation extending substantially perpendicular to the rulings, and the second adjuster provides for relatively adjusting the rulings about the second orthogonal rotational axis in an orientation extending substantially normal to the planar surface of the diffractive optic.
For defining the pivot axis, the flexural member is preferably mounted between nominally parallel mounting surfaces of the mounting arm and the laser mount. The pivot axis extends parallel to the nominally parallel mounting surfaces of the mounting arm and the laser mount. However, the first adjuster can provide for making small angular adjustments between the nominally parallel mounting surfaces of the mounting arm and the laser mount about the first orthogonal axis to better align the rulings of the diffractive optic substantially parallel to the pivot axis.
The flexural member in its preferred form has a plate-shaped body, and the pivot axis is defined by the bending of the plate-shaped body. For exerting the desired preload torques, the plate-shaped body of the flexural member is preferably made of a resilient material. A first fastening system preferably connects the plate-shaped body of the flexural member to the laser mount, and a second fastening system preferably connects the plate-shaped body of the flexural member to the mounting arm. One of these first and second fastening systems is preferably adjustable for adjusting the relative angular orientation of the mounting arm with respect to the laser mount about the first orthogonal rotational axis. The pivot axis extends substantially parallel to a plane of the plate-shaped body of the flexural member, and the first orthogonal rotational axis extends normal to the plane of the plate-shaped body of the flexural member.
An adjustable external cavity arrangement for a wavelength tunable laser according to the invention includes a laser mount for supporting the wavelength tunable laser, which emits an output beam whose peak wavelength is adjustable by wavelength feedback within the external cavity. A mounting arm supports both a diffractive optic and a reflective optic for movement with the mounting arm. A pivot supports relative rotation between the laser mount and the mounting arm for orienting the diffractive optic with respect to the laser through a range of pitch angles. The diffractive optic is movable with respect to the laser through the range of pitch angles for providing feedback to the laser as a corresponding range of wavelengths. The reflective optic is movable together with the diffractive optic for maintaining a substantially fixed angular orientation of the output beam with respect to the laser mount as reflected from the reflective optic. The diffractive optic and the reflective optic have front and back surfacesxe2x80x94the front surface of the diffractive optic being an operative surface for diffracting the output beam, and the front surface of the reflective optic being an operative surface for reflecting the output beam. The mounting arm has first and second mounting surfaces. The front surface of the diffractive optic is mounted against the first mounting surface of the mounting arm, and the front surface of the reflective optic is mounted against the second mounting surface of the mounting arm for mounting the diffractive optic and the reflective optic for movement together with the mounting arm.
As so mounted, the first and second mounting surfaces preferably have a fixed relation to each other and a fixed relation to the mounting arm. Preferably, the first and second mounting surfaces are fixed parallel to each other. In a preferred form of the mounting arm, a common aperture is formed through the first and second mounting surfaces for conveying the output beam between the diffractive optic and the reflective optic. The common aperture extends beyond one side of the reflective optic to permit the output beam inside the external cavity to reach the diffractive optic, and the common aperture extends beyond one side of the diffractive optic to permit the output beam outside the external cavity to propagate past the diffractive optic.
An alignment system for an external cavity of a laser in accordance with the invention includes a laser mount for the laser, which emits a wavelength-tunable output beam from the external cavity, and a mounting arm carrying a diffractive optic for providing wavelength feedback to the laser. A pivot permits a range of relative rotations between the mounting arm and the laser mount about a pivot axis for varying the wavelength of feedback to the laser. An adjuster adjusts a relative angular orientation of the mounting arm with respect to the laser mount about an adjustment axis that is oblique to the pivot axis. The mounting arm has first and second junctures that are spaced apart along a length of the mounting arm on opposite sides of the diffractive optic. The first juncture is connected to the pivot for supporting the range of relative motions between the mounting arm and the laser mount about the pivot axis. The second juncture is connected to the adjuster for adjusting the relative angular orientation of the diffractive optic for aligning the wavelength feedback to the laser.
The adjuster is preferably a second of two adjusters, and the adjustment axis is preferably a second of two mutually oblique adjustment axes. The first of the two adjusters provides for adjusting the relative angular orientation of the mounting arm with respect to the laser mount about the first of the two mutually oblique adjustment axes. Preferably, the first adjuster is connected to the first juncture of the mounting arm for further adjusting the relative angular orientation of the diffractive optic for aligning the wavelength feedback to the laser.
The diffractive optic preferably has a planar surface with rulings, and the first and second adjusters provide for relatively adjusting the rulings substantially parallel to the pivot axis. The adjustments of the two adjusters can be made independently by orienting the oblique adjustment axes mutually orthogonal. The first adjuster preferably provides for relatively adjusting the rulings about the first adjustment axis in an orientation extending substantially perpendicular to the rulings, and the second adjuster preferably provides for relatively adjusting the rulings about the second adjustment axis in an orientation extending substantially normal to the planar surface of the diffractive optic.
An actuator pivots the mounting arm with respect to the laser mount about the pivot axis for varying the orientation of the diffractive optic with respect to the laser. The pivot is preferably formed by a flexural member that exerts a preload force on the actuator. The flexural member also preferably exerts a preload force on the second adjuster. The flexural member preferably has a plate-shaped body, and both the pivot axis and the second adjustment axis preferably lie substantially in a plane of the plate-shaped body of the flexural member. In addition, the flexural member preferably provides resistance to relative rotation between the mounting arm and the laser mount about the first adjustment axis that extends normal to the plane of the plate-shaped body of the flexural member. The preferred mounting arm has a home position for tuning the laser to a predetermined wavelength, and the flexural member exerts a preload force between the mounting arm and the actuator at the home position.
The alignment system also includes a base supporting both the laser mount and the adjuster. The second adjuster adjusts the second juncture of the mounting arm with respect to the base for varying the relative angular orientation of the diffractive optic with respect to the base. The second adjuster preferably includes a guideway that restricts motion of the mounting arm with respect to the laser mount in a direction parallel to the pivot axis while permitting motion of the mounting arm with respect to the laser mount in a plane normal to the pivot axis. The second adjuster also preferably provides for adjusting the guideway parallel to the pivot axis. The mounting arm is preferably preloaded against the guideway.
The actuator referred to above is preferably an electrical device such as a solenoid or an electric motor. Preferably, associated with the actuator are a position transducer and a position encoder for determining the position of actuator arm. A microprocessor responsive to commands from a computer system preferably controls the actuator for adjusting the wavelength output of the external cavity laser as required for its intended use. Generally, the external cavity contemplated for the invention has a Littrow configuration in which the first diffraction order is returned directly to the laser. However, the invention can also provide benefits for other cavity configurations. The descriptions of the invention are exemplary in nature for calling attention to specific forms of the invention to aid in its practice and understanding.